COMMUNICATION APPARATUS FOR CODE DIVISION MULTIPLE ACCESSING MOBILE COMMUNICATION SYSTEM
1. Field of the Invention
The present invention relates to a communication apparatus used for a code division multiple accessing (CDMA) mobile communication system.
2. Description of the Related Art
Conventionally, a matched filter or a correlator has been used to demodulate a received spread spectrum signal in a code division multiple accessing (CDMA) mobile communication system. In this case, a transmitted signal from a transmitter side communication apparatus is received by a receiver side communication apparatus via a plurality of paths. Such a superimposed multi-path signal is entered to the matched filter, then a signal corresponding to a received signal intensity of the multi-path signal is output. FIG. 20 shows a communication apparatus which uses a conventional matched filter. A signal received in a carrier frequency band at a receiving antenna 2305 of a receiver side communication apparatus 2300-2 is converted to a base band spread spectrum signal by a radio frequency demodulator 2306, then converted to a digital signal by an A/D converter 2307. The base band digital spread spectrum signal is then despread by a matched filter 2308. The matched filter 2308, used for searching a path, outputs a peak value at timing synchronized with the multi-path signal. The signal from the matched filter 2308 is entered to a peak detector 2309. The peak detector detects a predetermined number of timings at which peak values have a large receiving intensity, then sets the phase of each of spreading codes of the despreaders 2310-1 to n at each peak timing. Each of the despreaders despreads a spread spectrum signal at timing specified by the peak detector 2309 and outputs a signal of a symbol rate. In the despreader 2310, the multiplier 2351 multiplies the spread spectrum signal with a spreading code generated by the code generator 2350, and the adder 2352 adds up the values and the register 2353 accumulates the total value during one symbol period. The signal output of a symbol rate from each despreader 2310 is entered to a rake combiner 2311 so that its phase is adjusted, then the rake-combined signal is output.
The receiver side communication apparatus shown in FIG. 20 performs antenna diversity. Antenna diversity means combining signals received by antennas 2305-1 to k disposed at predetermined pitches, thereby a diversity effect obtained. The signal received at each antenna is processed in the radio frequency demodulator, the A/D converter, and the base band demodulator sequentially, then subjected to diversity combination in the antenna diversity combiner 2338.
In the structure of the communication apparatus shown in FIG. 20, the matched filter is used for path searching and the despreader is used for despreading the spread spectrum signal. In this case, path searching is not always done, but performed at fixed intervals so as to prevent the signal from out of synchronization, thereby adjusting the phase at the despreader 2310.
For such the antenna diversity performed with the use of a matched filter in a conventionalstructure, the antenna diversity has required individual circuits for path diversity combining and antenna diversity combining respectively. Adding to antenna diversity state, when diversity handover state occurs and QPSK is employed as a radio demodulation method, a plurality of received signals must be demodulated simultaneously. In such a case, the conventional configuration has been confronted with a problem that the circuit is expanded significantly in scale.
In order to prevent such a significant circuit expansion, such a circuit as a matched filter has been time-divided for performing path searching. In the case of such a conventional technique, a plurality of received signals or spreading codes are selected sequentially at fixed periods, thereby searching the path of each of a plurality of channels. In this case, however, if the number of path search processings (number of antennas, number of channels, etc.) is increased, the assigned path searching period of each signal also becomes longer, so that the path searching cannot follow up with the time variance of the connected line. Because the number of received signals or spreading codes which can be time-divided is limited such way, a plurality of matched filters have been required and resulted in an expansion of the hardware in scale.
Furthermore, because the despreader must be kept active so as to despread each spread spectrum signal and demodulate the data, the despreader has to be prepared as many as the number of receiving channels, thus resulting in an expansion of the hardware in scale.
Under the circumstances, it is an object of the present invention to reduce such the hardware scale by operating a matched filter or a despreader in a time-division multiplexing manner.
In order to achieve the above object, the present invention provides two methods for a time-division-multiplexing processing of a matched filter.
(1) Time Division Multiplexing of Received Base Band Signals
A plurality of received base band signals are multiplexed in a time division manner, then the time-division-multiplexed signals are subjected to a despreading processing according to an operation clock decided by the number of time division multiplexing processings.
(2) Time Division Multiplexing of Spread Signals
A receiver generates a plurality of spreading codes for a received base band signal, then those spreading codes are selected by an operation clock decided by the number of codes, thereby despreading a plurality of the signals.
Those two methods are independent of each other, and both or either of them can be employed for a matched filter. With such time division multiplexing processings performed as described above, the performance of the matched filter can be improved, thereby reducing the hardware in scale.
Furthermore, because such the time division multiplexing processings are done even in the despreader, the number of despreaders disposed in parallel can be reduced.
As described above, the present invention allows both of a matched filter and a despreader to be shared with a plurality of channels. The matched filter can also share more channels than the conventional method. Furthermore, because the time division multiplexing processing as described above can be also performed on the transmission side, thereby allowing the spreader to be shared by a plurality of channels and reducing the hardware in scale.